Progress on the Dexter Hybrids
By David G. Leach, Brookville, Pa.
Guy Nearing's fine article in the October issue of the Bulletin indicates that he and perhaps many others would be interested to know the progress that has been made in isolating, testing and arranging for the introduction of the best Dexter hybrid rhododendrons by the Committee which has undertaken the task.
I did not have the privilege of knowing Mr. Dexter when he was active as a hybridist but his friends say that he raised a minimum of 20,000 rhododendrons from his crosses each year so there must have come from his talented hands a total of more than two hundred thousand hybrids. In addition, he gave away to others vast quantities of seedlings in flats. From these original hybrids second and third generations have since been produced, usually by professional growers for sale as unselected seedlings, and there now exist uncounted multitudes of rhododendrons which are known as Dexter hybrids. Nearly all of them by their indifferent quality debase a name which should be honored for its contribution to American horticulture.
In his lifetime Mr. Dexter made a few selections from among his legions in the fields and two or three of his friends were presented with plants propagated as layers from these fine hybrids. As the seedlings which he gave away in flats matured, and as older plants which were sold to enthusiasts by Mr. Dexter showed their worth, a few more became known among a handful of hobbyists for their superior quality, notably in several collections on Long Island. But these fine specimens remained in private gardens. They were unknown to the public while unselected seedlings gained popularity in commerce as nurserymen promoted them on an unearned reputation for being much better than the standard Catawba hybrids. Thus Mr. Dexter's name has gradually become associated, not with the best of his productions as is the case with other hybridists, but with the worst. This is manifestly unjust since he produced just as large a proportion of truly fine things from his efforts as have other breeders, and it is horticulture's loss that these outstanding rhododendrons have never been distributed to the public.
In 1950 a voluntary committee was formed under the leadership of John Wister, Director of the Arthur Hoyt Scott Horticultural Foundation at Swarthmore College, Swarthmore, Pa., with the idea of selecting the very best Dexter hybrids, testing them side by side at Swarthmore and elsewhere, and then arranging for the introduction of the finest of each flower color, stature and season of bloom. An annual tour began, inspecting the noteworthy collection all the way from Winterthur, Delaware to the border of New Hampshire.
The very next year the original Dexter collection at Sandwich, Massachusetts consisting of more than 15,000 rhododendrons 10 to 15 feet tall was completely broken up and sold to hundreds of different buyers all over the East. Some good hybrids found their way into the hands of amateurs who later offered cuttings to the Committee for its trials at Swarthmore, but no doubt many fine specimens thereby passed beyond our knowledge.
In 1951, however, the Committee repeated its tour and each year it has continued to visit some of the noteworthy collections, both private and public, to evaluate the superior hybrids and choose those to be tested. As selections have been made during the annual tours arrangements have been made to secure cuttings from the owners. These have been rooted by commercial propagators and the small plants have then been sent to Swarthmore for the comparative testing program.
It is impossible to judge accurately the original plants of these superior Dexter hybrids as they now grow in the various collections. Some of them receive good care whereas others are unfairly penalized by neglect. Some are planted in sites which are obviously not suited to their needs. Still others appear to be growing with difficulty at the very northernmost limit of their ability to survive the winter temperature. They must all be propagated and plants then grown side by side under identical cultural conditions to assess them in a comparative way.
It has, of course, taken a few years merely for plants propagated from the early selections to grow to flowering size and the later selections are not yet of blooming age at the trial grounds. But that is only the beginning of the time-consuming process. There is the matter of floriferousness and of hardiness, both qualities which take years to judge. The Committee does not want to inadvertently introduce a hybrid which may flower much less freely than another almost identical in blossom, habit of growth and season of bloom which it also has under test.
An example which illustrates the dilemma is a Dexter hybrid which I had admired for many years at the Parker estate on Long Island, where it was identified as PK #3. This plant had glossy green foliage of pleasing color, good growth habit and large, luminous dark rose flowers of about the same shade that has made 'Queen Wilhelmina' a favorite in milder climates. When the Parker collection was broken up the Coe estate acquired this plant and it was given the name 'Mrs. W. R. Coe'. From that source the Committee acquired a few cuttings. A year or two later the Committee came across a plant in a Massachusetts collection which is almost identical. It is probably hardier, however, and it may be more free blooming. If so, it should be chosen for introduction over the Long Island selection. But it will take a few years to tell. There is no short-cut to an accurate assessment of either floriferousness or cold tolerance. Only the passage of time and the advent of severe winters can demonstrate these qualities. If a hasty choice is made, nurserymen who propagate the first selection in good faith will be unjustly and severely penalized should a similar hybrid notably superior in hardiness and freedom of bloom be released.
The hybrid from the Everitt estate called 'C. O. D.' (Mr. Dexter's initials) has been sparingly distributed as plants given away by Mr. Everitt have been re-propagated. It has a champagne colored flower flushed amber-yellow and pink, an unusual and effective combination. It has only five or six blossoms in the truss, however, and its foliage is both poor in color and deficient in quantity. It is possibly the best known of all the Dexter hybrids and it has been listed in the catalogue of at least one nurseryman, the first Dexter hybrid to be offered for sale as a clone, to my knowledge. Fortunately for enthusiasts, but unfortunately for nurserymen who may be busily propagating 'C. O .D.', the Committee is testing plants of another hybrid almost identical in flower color but far superior in foliage and habit, and probably a good deal hardier as well. The Committee wishes to avoid responsibility for hardships on professional growers arising from its own selections, but it can do so only by taking the long time necessary to evaluate correctly the plants it is testing.
Before they invest time and money in propagating an unknown Rhododendron hybrid, nurserymen do want to know just how hardy it is. Many growers have suffered heavy losses after a severe winter has left their fields blighted by extensive cold injury. The climate at the main testing grounds at Swarthmore is relatively mild, however, and survival there does not insure satisfactory performance at Boston or even at New York City. The only way the limit of cold endurance can be determined is to re-propagate the Swarthmore selections and repeat the trials in progressively colder climates. This is being done. Plants have already been sent to the Arnold Arboretum, to the National Arboretum and to a test site on Long Island. But it all takes time a long time.
There is one group of Dexter hybrids which has already proved its adaptability for cultivation on a really wide scale in the northeastern United States. It is in a collection open to the public in the display garden of the Bosley Nursery at Mentor, Ohio. Mr. Bosley went to Sandwich, Massachusetts the year before Mr. Dexter died and he selected a full railroad carload of plants designated by Mr. Dexter on the basis of their parentage to be extra-hardy. After they were planted out in the more severe climate at Mentor the majority died in the course of the ensuing winters but finally a small group of hardy survivors remained which were just as tough there as the standard Catawba hybrids. They have subsequently been tested in a climate even a good deal colder and most of them have been satisfactory.
As a group, the Dexter hybrids at Mentor are remarkable for their clarity of color in plants so hardy, and for their extraordinary tolerance of exposure. They are growing without any overhead shade whatever and only one of the group shows any evidence of resenting this open-field treatment, which results in unbelievably abundant flowering and handsome, compact growth. The flowers are notably larger than those of the standard hybrids in commerce, though their average size is somewhat smaller than that of the best Dexter hybrids in the less hardy group taken from the collections on the Atlantic seaboard. The hybrids in the East are also more fragrant.
The Dexter hybrids at Mentor are an important step forward in the evolution of Rhododendron hybrids of ironclad hardiness. Mr. Bosley has named two of the best 'Brown Eyes' and 'Lavender Princess'.
West Coast growers find it hard to grasp the excitement of eastern enthusiasts over the best of the Dexter hybrids. They are unfamiliar with the limited color range, the small flowers and stiff, formal trusses of the older Waterer hybrids which have been typical of the only rhododendrons in commerce in the East for three quarters of a century. When the new hybrids started to come from Mr. Dexter they represented a radical departure from the conventional rhododendrons in our gardens. Their large flowers, many of them fragrant, were produced in refreshing clear colors of great clarity and charm, devoid of the abrasive blue taint which had characterized their predecessors. Not only were the flowers two or three times larger than those of the Waterer hybrids, but they were produced a couple of weeks earlier and so the blooming season was correspondingly lengthened. The foliage was larger and better in color on many of the Dexter hybrids and the trusses were informal clusters in which the flowers were often held gracefully and individually for the best display of their form. Some fanciers praised the novel truss formations. Others condemned them as a lamentable fault in otherwise estimable plants.
Thus the Dexter hybrids did represent a new standard of quality for the cold Northeast and they added a new dimension to the conception of garden rhododendrons. From close observation of many thousands of plants it is my personal belief that a hybrid called 'Farquhar's Pink' which Mr. Dexter obtained from the Farquhar Nursery on Cape Cod, figured prominently in the parentage of many of his crosses. It is popularly thought that R. fortunei contributed mainly to the large flower size characteristic of this group but the plants with stamens hairy at the base and a style glandular at the tip could only have been derived from R. decorum. We can only speculate because Mr. Dexter left no records.
The vast majority of Dexter hybrids as they are seen in commerce today have reverted to the lowest common denominator of their mixed parentage. Their flowers are a faded lavender in color and deficient both in size and in the number of them to the truss. Their growth habit is a little coarse and they vary enormously in hardiness. The Committee which has been trying to isolate and eventually arrange for the introduction of a handful of truly superior clones has been made up of Clement Bowers, Henry Skinner, Paul Vossberg, Donald Wyman and the writer, with John Wister as de facto chairman bearing by far the heaviest burden of the work. Its task is still a long way from conclusion.